1. Field of the Invention
The present invention relates to the detection of whether a signal read from an optical disc has a defect, and more particularly, to an apparatus and method accurately detecting a defect signal regardless of the magnitude of an input signal.
2. Description of the Related Art
In general, data is read from optical discs by radiating laser light onto the surfaces of the optical discs and then converting the strength of the laser light reflected from the optical discs into an electrical signal. The electrical signal is called a radio frequency (RF) signal. The RF signal is converted into a binary signal, the binary signal is demodulated, and the demodulated signal is used to reproduce user data. However, the surface of an optical disc is frequently scratched or covered with dust, resulting in an error in the produced RF signal.
In order to read data from several parts of an optical disc, a laser may jump to a desired position of the optical disc instead of sequentially reading the optical disc from the inner perimeter to the outer perimeter. When jumping positions, the magnitude of the RF signal is reduced so as to be less than the magnitude when the laser performs a normal reading operation. Accordingly, if an abnormal RF signal is input, a signal indicating the abnormality of the RF signal is referred to as a defect signal.
FIG. 1 illustrates the structure of a conventional defect signal detecting apparatus. The conventional defect signal detecting apparatus includes a peak holding circuit 101, which follows only a peak level of an input RF signal, and a bottom holding circuit 102, which follows only a bottom level of the input RF signal. The peak holding circuit 101 and the bottom holding circuit 102 generally use filters. If the peak level and the bottom level of the RF signal are detected, a first comparator 103 compares the peak level with a predetermined peak threshold level and a second comparator 104 compares the bottom level with a predetermined bottom threshold level. If the peak level and the bottom level are greater than or smaller than the predetermined threshold levels, the RF signal is detected as a blank signal.
According to the prior art, since a peak holding circuit following only a peak level of an input RF signal and a bottom holding circuit following only a bottom level of the input RF signal use filters, the peak holding circuit and the bottom holding circuit cannot rapidly follow the RF signal. If the peak holding circuit and the bottom holding circuit rapidly follow the RF signal by increasing a threshold frequency of the filters, the increased threshold frequency interferes with a long-period signal band included in the RF signal. Therefore, a defect signal is not properly detected, and as a result, it is difficult to detect an accurate defect signal at an accurate point of time.
In addition, if the strength of the RF signal shifts due to spots on the surface of the optical disc or a difference in the reflectivity of the optical disc, the peak holding circuit and the bottom holding circuit fix the threshold level. Thus, a defect in the RF signal is not properly detected due to the sharp change in the strength of the RF signal.